Semiconducting carbon nanotubes(CNTs) have several properties that are advantageous for field effect transistors such as high mobility, good electrostatics due to their small diameter allowing for aggressive gate length scaling and capability to withstand high current densities. However, in spite of the exceptional performance of single transistors only a few simple circuits and logic gates using CNTs have been demonstrated so far. One of the major obstacles for large scale integration of CNTs is to reliably fabricate p-type and n-type ohmic contacts. To achieve this, the nature of Schottky barriers that often form between metals and small diameter CNTs has to be fully understood. However, since experimental techniques commonly used to study contacts to bulk materials cannot be exploited and studies often have been performed on only single or a few devices there is a large discrepancy in the Schottky barrier heights reported and also several contradicting conclusions. This paper presents a comprehensive review of both theoretical and experimental results on CNT-metal contacts. The main focus is on comparisons between theoretical predictions and experimental results and identifying what needs to be done to gain further understanding of Schottky barriers in CNT-metal contacts.

Semiconducting carbon nanotubes(CNTs) have several properties that are advantageous for field effect transistors such as high mobility, good electrostatics due to their small diameter allowing for aggressive gate length scaling and capability to withstand high current densities. However, in spite of the exceptional performance of single transistors only a few simple circuits and logic gates using CNTs have been demonstrated so far. One of the major obstacles for large scale integration of CNTs is to reliably fabricate p-type and n-type ohmic contacts. To achieve this, the nature of Schottky barriers that often form between metals and small diameter CNTs has to be fully understood. However, since experimental techniques commonly used to study contacts to bulk materials cannot be exploited and studies often have been performed on only single or a few devices there is a large discrepancy in the Schottky barrier heights reported and also several contradicting conclusions. This paper presents a comprehensive review of both theoretical and experimental results on CNT-metal contacts. The main focus is on comparisons between theoretical predictions and experimental results and identifying what needs to be done to gain further understanding of Schottky barriers in CNT-metal contacts.

Raman spectra of pure pyridine, pyridine aqueous solution, and pyridine in methanol under high pressure were measured separately. Behaviors of two Fermi doublets, v1 and v12, v1+v6 and v8, occurred simultaneously in one pyridine molecule are analyzed according to their spectra, which indicates that the v1 Raman activity decreased with increasing pressure and disappeared eventually, which induced weakness and even disappearance of the Fermi resonance between v1 and v12, while the v1 Raman intensity variation had no effect on the presence of the Fermi resonance between v1+v6 and v8 as well as its variation law with increasing pressure. Those phenomena were interpreted by group theory in this article. It also indicates experimentally that all Raman bands of pyridine appeared blueshift with increasing pressure except that OH group appeared redshift. Moreover, frequency v1 shifted more quickly with increasing pressure than their counterparts did in neat liquid, so did its Raman intensity variation.

A critical point model with three Lorentzian terms for interband transition was proposed to describe temperature-dependent reflectivity (R) and absorption coefficient (α) for copperirradiated by ultrashort-pulsed lasers of wavelength 200–1000 nm. After validated with experimental data at room temperature, it was incorporated into a two-temperature model to study ultrafast laser-material interactions. The dynamic changes of optical propertiesR and α, distributions of laser heat density, electron and lattice temperature, and phase changes of a copperfilm were investigated. Comparing with the experimental data of average absorption showed that the proposed two-temperature model together with the critical point model can simulate satisfying results for temperature-dependent R and α. The drastic changes in R and α could alter laser energy deposition in a heated target, leading to different thermal responses than those predicted with constant R and α at room temperature.

It has fundamental meaning to find the elements influencing the laser-induced damage threshold (LIDT) of KH2PO4(KDP) crystal and to provide suitable characterization parameters for these factors in order to improve the LIDT of KDP. Using single-point diamond turning (SPDT) to process the KDP crystal, the machined surface quality has important effects on its LIDT. However, there are still not suitable characteristic parameters of surface quality of KDP to correspond with the LIDT nowadays. In this paper, guided by the Fourier model theory, we study deeply the relationship between the relevant characteristic parameters of surface topography of KDP crystal and the experimental LIDT. Research results indicate that the waviness rather than the roughness is the leading topography element on the KDP surface machined by the SPDT method when the LIDT is considered and the amplitude of micro-waviness has greater influence on the light intensity inside the KDP crystal within the scope of dangerous frequencies between (180 μm)−1 and (90 μm)−1; with suitable testing equipment, the characteristic parameters of waviness amplitude, such as the arithmetical mean deviation of three-dimensional profile Sa or root mean square deviation of three-dimensional contour Sq, are able to be considered as suitable parameters to reflect the optical quality of the machined surface in order to judge approximately the LIDT of the KDP surface and guide the machining course.

We present results of dynamic and fast switching of birefringence in a photochromicliquid-crystalline system as a function of the sample temperature. The system consists of photochromic molecules of 4-heptyl-4′-methoxyazobenzene showing a liquid-crystalline nematic state close to room temperature. An experiment of dynamic birefringence switching was done in optical Kerr-effect set-up, where for the sample excitation, a picosecond-pulsed laser was used. Measurements were done for different temperatures of the sample in the liquid-crystallinenematic phase. We have proposed a mathematical model of dynamic, fast, and fully reversible birefringence changes. Theoretical estimations and experimental results have shown very good agreement.

1 μm thick Si solar cells based on nanocone grating (NCG) with height of 100-800 nm and period of 100, 500, and 800 nm are numerically investigated through reflectivities,absorption enhancement factors, absorption spectra, optical generation rates, ultimate efficiencies, and diffraction angles. Compared with the planar Si solar cell,absorption enhancement are observed in any solar cells with NCG surface. Their absorption enhancement mechanism varies with the incident wavelength range. When incident wavelength λ < 500 nm, antireflection of their front surface dominates the absorption enhancement behavior due to their stronger absorption coefficients. When 600 nm > λ > 500 nm, even though the absorption enhancement is still dominated by antireflection of the front surface, cavity-resonance effect and guided-mode excitation induced by high order diffraction start to make contribution. When λ > 600 nm, the contribution of guided-mode excitation induced by lower-order diffraction becomes larger and larger once the diffraction angle is larger than its critical angle. For the structure with P = 100 nm, high-order diffraction cut-off at the longer wavelength range is the main reason of its lower absorption enhancement and ultimate conversion efficiency. For P = 800 nm, the lower absorption enhancement and ultimate efficiency is also observed due to the high reflection loss and mode leakage induced by 1st order diffraction where its diffraction angle is lower than its critical angle. Higher absorption and ultimate conversion efficiencies are achieved in P = 500 nm due to the good balance between antireflection performance and guide-mode excitation induced by the high order diffraction is achieved. Moreover, such absorption enhancement is closely related with its height of NCG gratings.Reflection loss reduction, the interaction volume reduction between the incident light and Si material, and higher photon density in NCG structure coexists with H increasing, which results in absorption enhancement in P = 500 nm and P = 800 nm, but absorption reduction in P = 100 nm where high order diffraction cut-off. Based on these analysis, we do believe that high absorption and ultimate conversion efficiency should be achieved in NCG-based solar cells where both the lower reflection in short wavelength domain and guide-mode excitation induced by 1st and 2nd diffraction in longer wavelength domain can be achieved. According to this rule, the optimized structure is NCG with P = 559 nm and H = 500 nm, by which, the highest optical generation rate of 536.57 × 104 W/cm3 and ultimate efficiency of 28.132% are achieved. Such analysis should benefit the design of the thin filmsolar cells with nano-structured diffraction gratings.

Poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)]}:PCBM bulk heterojunction solar cells were fabricated and characterized under different incident light power intensities. Charge-trapping effects take place at low fullerene content in the photoactive blend; an efficient polymerfullerene intermixing with formation of continuous phases is reached at a donor:acceptor ratio of 1:4. For an optimized active layer thickness of 100 nm, a power-conversion efficiency of 2.57% was obtained. Photocurrent measurements under reverse-bias conditions show that a high percentage of the photogenerated excitons does not lead to the formation of free carriers, thus representing the major limiting factor for the device’s efficiency.

Chloro-sulfide glass with low phonon energy, GeS2–Ga2S3–CsCl, is co-doped with Er and Yb. This active glass is a potential downconversion material for modifying the solar spectrum to improve the efficiency of solar cells. Two downconversion processes from visible to near infrared are observed. In the first process, an energy transfer between Er3+ions and Yb3+ions occurs. In that case, one photon is absorbed by the 4I15/2→2H11/2 (Er3+) transition and then two photons are emitted by 2F5/2→2F7/2 (Yb3+) and 4I13/2→4I15/2 (Er3+), respectively. In the second process, downconversion takes place from the charge-transfer state of Yb3+–S2− to the 4f states of Yb3+ions, which leads to an intense excitation band between 400 nm and 600 nm, and an emission at 1000 nm. Quantum yields for downconversion are measured. The highest quantum yields of emission below 1200 nm and 1650 nm are equal to 51% and 76%, respectively.

We present a study of the photo-excited charge carriersrelaxation dynamics in polar semiconductors comparing calculations to pump probe experiments.Hot carrier densities in the range can easily be photo-generated using moderately intense optical excitations. This can lead to known phenomena, namely, hot phonon populations and the coupling of polar optical phonons with plasmon modes. However, these two phenomena can affect the hot carriersrelaxation and have never been examined together. This is a problem for the theoretical study of future Hot Carrier Solar Cells, where the conditions allow both of these phenomena to occur. The charge carriers dynamics and the coupling of polar optical phonons with plasmon modes are treated by a Full Band Ensemble Monte Carlo simulation code featuring a self-consistent dielectric function. To take into consideration hot phonon populations and the subsequent phonon bottleneck for the carriers relaxation, the charge carriers simulation code is coupled to a phonon dedicated Ensemble Monte Carlo code. This enables for the first time an accurate study of both the charge carriers and phonon systems dynamics, the latter being most of the time overly simplified in previous studies. The present work explores to which extent the two aforementioned phenomena affect the photo-generated charge carriersrelaxation in GaAs and can be easily adapted to other polar semiconductors.

In this study, efficient input and output power coupling schemes for transition regions at the interface of conventional and slow light waveguides are investigated. By optimizing the tapered nano-tip of the input and output slab waveguides that support a group index of 3.58, we achieved 97% coupling efficiency to a square-lattice based slow light photonic crystal waveguide with a group index of 1200. The complementary slow waveguidestructure based on triangular-lattice is also designed to support same order of magnitude slow light mode and targeted to alleviate the severity of the coupling loss. An acceptable efficiency value is recorded for the second type of slow waveguide mode. For the sake of targeting only input and output coupling losses, we made an assumption that other loss mechanisms are absent in the structure. The successful demonstration of effective and compact slow light couplers will assist the deployment of slow light devices in important applications, such as nonlinear optics, optical buffers, and optical delay lines.

The spontaneous emission characteristics of green- and red-emitting InGaNquantum wells(QWs) on ternary InGaN substrate are analyzed, and the radiative recombination rates for the QWsgrown on ternary substrate were compared with those of InGaNQWs on GaN templates. For green- and red-emitting InGaNQWs on In0.15Ga0.85N substrate, the spontaneous emission rates were found as ∼2.5-3.2 times of the conventional approach. The enhancement in spontaneous emission rate can be achieved by employing higher In-content InGaN ternary substrate, which is also accompanied by a reduction in emission wavelength blue-shift from the carrier screening effect. The use of InGaN substrate is expected to result in the ability for growingInGaNQWs with enhanced spontaneous emission rates, as well as reduced compressive strain, applicable for green- and red-emitting light-emitting diodes.

Laser-induced-damage characteristics of commercial indium-tin oxide (ITO) filmsdeposited by DC magnetron sputteringdeposition on K9 glass substrates as a function of the film thickness have been studied at 1064 nm with a 10 ns laser pulse in the 1-on-1 mode, and the various mechanisms for thickness effect on laser-induced-damage threshold (LIDT) of the film have been discussed in detail. It is observed that laser-damage-resistance of ITO film shows dramatic thickness effect with the LIDT of the 50-nm ITO film 7.6 times as large as the value of 300 nm film, and the effect of depressed carrier density by decreasing the film thickness is demonstrated to be the primary reason. Our experiment findings indicate that searching transparent conductive oxide (TCO)film with low carrier density and high carrier mobility is an efficient technique to improve the laser-damage-resistance of TCOfilms based on maintaining their well electric conductivity.

We report our experimental studies on time-resolved pump–probe spectroscopy in high-quality Al0.86Ga0.14N single crystals,grown using a solution technique in a high-nitrogen-gas-pressure system. Our optical measurements were performed using a non-traditional, two-beam [one ultraviolet (UV) and one infrared (IR)], femtosecond pump–probe approach, in which the photon energies of both beams were below the bandgap of the sample and each electron–hole pair was generated by a multi-photon process of absorption of a pump photon together with another photon produced by second harmonic generation from two probe photons. Temporal scanning of the probe while monitoring the normalized transient differential transmissivity (ΔT/T) signal, produced a 310-fs-wide, Gaussian-shaped correlation signal caused by the multi-photon absorption process, followed by a >100-ps-long relaxation of photo-excited carriers. By studying the ΔT/T correlation signal amplitude dependence on the pump-power intensity and wavelength, the multi-photon absorption was determined to be predominantly caused by absorption of a pump photon and a second harmonic photon from the probe.

Homogeneous NaYF4:Yb3+/Er3+ nanoparticles (NPs) with average diameters of ∼10 nm and ∼200 nm and various doping concentrations (Yb3+:0%−20%, Er3+:2%) were prepared by the thermal decomposition of trifluoroacetate precursors. The visible and infrared (IR) emission spectra range of 500–2200 nm and luminescent dynamics were studied through the pumping of multi-wavelengths, 443 nm, 488 nm, and 520 nm. Strong and sufficient IR emissions were observed, including the transitions of 4I11/2−4I15/2 at ∼980 nm, 2H11/2−4I11/2 at ∼1112 nm, 4S3/2−4I11/2 at ∼1217 nm, 4I13/2−4I15/2 at ∼1540 nm, 4I9/2−4I13/2 at ∼1680 nm, and 4F9/2−,4I11/2 at ∼1955 nm. It is the first observation of 2H11/2−4I11/2 and 4F9/2−4I11/2 emissions to our knowledge. Through the IR emissions, several novel channels of quantum cutting (QC) were evidenced, including: (1) 2H11/2−4I11/2 and 4I11/2−4I15/2, (2) 4S3/2−4I11/2 and 4I11/2−4I15/2, (3) 4F9/2−4I11/2 and 4I11/2−4I15/2, and (4) 4I9/2−4I13/2 and 4I13/2−4I15/2. For the IR QC emissions, the overall efficiencies in the 200−nm NaYF4:Yb3+, Er3+ were estimated to be as high as 186−193%. Through the measurements of luminescent dynamics of Er3+ on different levels, the spontaneous rates and energy transfer (ET) rates from Er3+ to Yb3+ were determined, which showed that ET from Er3+ to Yb3+ mainly happened on 2H11/2,4S3/2, and 4I13/2 levels. The present results indicate that the visible-to-IR QC for Er3+ has potential use to improve the efficiency of some IR solar cells, such as germanium-based ones.

The forcing behavior of a dielectric barrier discharge (DBD) actuator is investigated experimentally using a time-resolved particle image velocimetry(PIV) system in conjunction with a phase shifting technique. The spatio-temporal evolution of the induced flowfield is accurately captured within one high voltage (HV) cycle allowing the calculation of the instantaneous velocity and acceleration. Additional voltage and current measurements provide the power consumption for each case. Four different applied voltage waveform shapes are independently tested, namely, sine, square, positive sawtooth, and negative sawtooth at fixed applied voltage (10 kVpp) and carrier frequency (625 Hz). The instantaneous flowfields reveal the effect of the plasma forcing during the HV cycle. Sine waveform provides large positive forcing during the forward stroke, with minimal but still positive forcing during the backward stroke. Square waveform provides strong and concentrated positive and negative forcing at the beginning of the forward and backward stroke, respectively. Positive sawtooth provides positive but weak forcing during both strokes while the negative sawtooth case produces observable forcing only during the forward stroke. Results indicate the inherent importance of negative ions on the force production mechanisms of DBD’s. Furthermore, the revealed influence of the waveform shape on the force production can provide guidelines for the design of custom asymmetric waveforms for the improvement of the actuator’s performance.

The dynamic of charged particles in pulsed plasma is relatively well known since the 1990s. In contrast, works reporting on the impact of the plasma modulation frequency and duty cycle on the radicals’ densities are scarce. In this work, we analyze the impact of these modulation parameters on the radicals’ composition in Cl2 and HBr plasmas. The radicals’ densities are measured by broad-band UV and vacuum-ultraviolet (VUV) absorption spectroscopy and modulated-beam mass spectrometry. We show that pulsing the rf power allows controlling the plasma chemistry and gives access to the plasma conditions that cannot be reached in continuous wave plasmas. In particular, we show that above 500 Hz, the pulsing frequency has no influence on the plasma chemistry, whereas in contrast the duty cycle is an excellent knob to control the fragmentation of the parent gas, thus the chemical reactivity of the discharge. At low duty cycle, a reduced gas fragmentation combined with a large ion flux leads to new etching conditions, compared to cw plasmas and the expected consequences on pulsed-etching processes are discussed.

Measurements with a rf compensated Langmuir probe and optical emission spectroscopy are carried out in capacitively coupled rf (13.56 MHz) pure nitrogen discharges at fixed rf voltage over a wide range of pressure, 30 to 400 mTorr. The electron energy probability function (EEPF) measured below 100 mTorr resembles a bi-Maxwellian-type distribution. At pressure range of 100-200 mTorr, the EEPF has non-Maxwellian distribution with a “dip” near 4.5 eV. At the highest pressure of 400 mTorr, the EEPF evolves into a Druyvestein-like distribution and the “dip” disappears. The electron density significantly decreases with increase in the pressure. On the other hand, the electron temperatures gradually decrease with an increase in pressure, reaching minimum at 150 mTorr, beyond which it abruptly increases. Such evolution of the EEPFs shape with gas pressure has been discussed in terms of non-local electron kinetics and heating mode transition. The emission intensities of nitrogen (0-0) band of second positive system at 337.1 nm and (0-0) band of first negative systems at 391.4 nm are used to determine the dependence of their radiative states and with nitrogen pressure. It is observed that the pressure influences the radiative states differently owing to their different populating mechanisms. The vibrational temperature and rotational temperature are measured for the sequence () of second positive system using the method of comparing the measured and calculated spectra with a chi-squared minimization procedure. It was found that both and have similar dependences with pressure; peaked at 100 mTorr beyond which it monotonically decreases with increase in the pressure. The correlation between the observed maximum value of around 100 mTorr and the detected “dip” in the EEPF in the same pressure range has been discussed.

We have developed a technique for absolute measurements of electron density in pulse-repetitive microwavedischarges in air. The technique is based on the time-resolved absolute intensity of a nitrogen spectral band belonging to the Second Positive System, the kinetic model and the detailed particle balance of the N2C3Πu ( = 0) state. This new approach bridges the gap between two existing electron density measurement methods (Langmuir probe and Stark broadening). The electron density is obtained from the time-dependent rate equation for the population of N2C3Πu ( = 0) using recorded waveforms of the absolute C3Πu → B3Πg (0-0) band intensity, the forward and reflected microwave power density. Measured electron density waveforms using numerical and approximated analytical methods are presented for the case of pulse repetitive planar surface microwavedischarge at the aperture of a horn antenna covered with alumina ceramic plate. The discharge was generated in air at 11.8 Torr with a X-band microwave generator using 3.5 μs microwave pulses at peak power of 210 kW. In this case, we were able to time resolve the electron density within a single 3.5 μs pulse. We obtained (9.0 ± 0.6) × 1013 cm–3 for the peak and (5.0 ± 0.6) × 1013 cm–3 for the pulse-average electron density. The technique presents a convenient, non-intrusive diagnostic method for local, time-defined measurements of electron density in short duration discharges near atmospheric pressures.

A numerical solution is obtained for the electron and ion number densities, and electric field of an rf argon plasma in a low pressure reactor utilizing a one-dimensional model. These variables are used to solve the equations describing the dynamical behavior of a dust particle under the influence of the electrical, gravity, and ion and neutral drag forces. The effects of the rf oscillations of the plasma on the dust particle are investigated through comparisons made between two sets of results. The first set is generated by a model in which the rf-period-averaged plasma variables are used in the dust particleequations while the second set is generated using the instantaneous plasma variables, without rf-period averaging. These two sets of results including the positions and charges of, and the various forces acting on the dust particles with different sizes and densities, are compared and significant differences are found.